The invention relates to a process for hydroprocessing hydrocarbon-containing oils.
During catalytic hydroprocessing, particulate catalysts are utilized to promote reactions such as desulfurization and denitrogenation. This is accomplished by contacting the particulate catalysts with a feedstock, such as a gas oil, under conditions of elevated temperature and pressure and in the presence of hydrogen so that the sulfur components are converted to hydrogen sulfide, and nitrogen components to ammonia. Hydroprocessing is typically employed to reduce the concentration of nitrogen and sulfur in feedstocks so as to produce hydrocarbons which, when eventually combusted, result in reduced air pollutants of the forms NO.sub.x and SO.sub.x. Reducing the concentration of nitrogen is also desirable to protect other refining catalysts, such as hydrocracking catalysts, which deactivate in the presence of nitrogen.
A typical hydroprocessing catalyst contains hydrogenation metals and/or other promoters on a porous refractory oxide support. Hydrogenation metals usually include Group VIB and/or Group VIII active metal components supported on a porous refractory oxide support such as alumina. Other promoters, such as phosphorus components, have also been incorporated in such catalysts. Such catalysts are often prepared by impregnation, that is, the deposition of the active components on the support base by contact thereof with an aqueous solution containing the active components in dissolved form. The impregnated supports are usually calcined, thus converting the promoters to the oxide form, and then the catalyst is activated for use.
An important and continuing aim--in the catalyst refining art--is to discover catalysts of improved activity and/or stability. Increasing the activity of a catalyst increases the rate at which a chemical reaction proceeds under given conditions, and increasing the stability of a catalyst increases its resistance to deactivation, that is, the useful life of the catalyst is extended. In general, as the activity of the catalyst is increased, the conditions required to produce a given end product, such as a hydrocarbon of given sulfur or nitrogen content, becomes more mild. Milder conditions require less energy to achieve a desired product, and the catalyst's life is extended due to such factors as lower coke formation, etc.
Modest or slight variations in compositional characteristics or methods of preparation of hydroprocessing catalysts have been known to have highly unpredictable activity or stability effects on hydrocarbon conversion reactions (such as denitrogenation and/or desulfurization reactions). Three such variable compositional characteristics are: (1) porosity characteristics of the catalyst derived from its porous refractory oxide support; (2) the actual hydrogenation metal promoters (Ni, Co, Mo, W, etc.) and other promoters (P, etc.) in the catalysts; and (3) the percentages of the promoters in the catalyst. Variations of catalyst preparation include impregnation, comulling, coprecipitation, and cogellation.
The petroleum refiner must balance economic considerations, such as the cost of catalyst preparation, with the catalyst characteristics affecting catalyst activity and/or stability. One group of hydroprocessing catalysts providing suitable service to petroleum refiners for hydrodenitrogenation (in terms of both activity and economics) contain nickel, molybdenum and phosphorus promoters (commonly called "Ni--P--Mo" catalysts) supported on porous refractory oxides having a wide variety of pore size distributions. Each variation in porosity can impart a significant variation in catalyst properties, even for Ni--P--Mo catalysts containing the same relative weight percentages of promoters. Similarly, small variations in the percentages of Ni--P--Mo promoters can alter catalyst properties substantially. Furthermore, slight modifications in catalyst preparation procedures, such as the manner of incorporating the Ni--P--Mo promoters with the refractory oxide supports, or the effective calcination temperature, can likewise unpredictably affect catalyst activity and/or stability properties.
A commercial Ni--P--Mo catalyst having a specific narrow pore size distribution and at least 24.5 weight percent of molybdenum components, calculated as MoO.sub.3, has been useful in hydroprocessing hydrocarbon oils. However, molybdenum is relatively expensive and its relatively high weight percentage contributes significantly to the costs of the commercial hydroprocessing catalyst, and ultimately to the cost of hydroprocessing with the catalyst. A relatively small reduction in the weight percentage of molybdenum on a catalyst can result in huge cost savings to the petroleum refiner. On the other hand, nickel is considerably cheaper than molybdenum, and an increase in the weight percentage of nickel on a catalyst becomes economically viable, provided activity and/or stability effects outweigh an increase in cost of manufacture of the catalyst. Accordingly, the petroleum refiner or catalyst manufacturer has a keen economic interest in a catalyst containing a reduced amount of costly molybdenum when improved catalyst activity and/or stability effects can be achieved with moderate increases in nickel content in the catalyst.